The fundamental techniques of xerographic printing are established and well known in the art. Basically, a latent image is formed on a charged photoreceptor by sweeping a modulated laser beam across the surface of the photoreceptor. This latent image is then used to create a permanent image by transferring and fusing toner, that is electrostatically attracted to the latent image, onto a recording medium, usually plain paper.
Extending xerography to include full color printing presents a set of unique problems. For example, the xerographic system must "know" which colors to apply to the recording medium in response to particular laser markings. Many methods have been devised in the current art to overcome this problem.
For example, some xerographic systems apply only one wavelength of light to the surface of the photoreceptor apparatus. The photoreceptor, in turn, interprets the single wavelength as a different color to be applied to the recording medium depending on the particular time or location the light reaches the photoreceptor. A typical system of this type uses a multi-pass method of producing full color prints. The system produces a single wavelength of light that passes across the surface of the photoreceptor several times--each pass interpreted as a separate color. To supply a different color per pass, a different colored toner is applied to the surface of the photoreceptor which is then transferred to the recording medium. This procedure is repeated until each desired color is laid down.
The main disadvantage of this system is speed. Executing multiple passes over the photoreceptor to produce one page of print is slow. Theoretically, if all the different colors could be laid down on the recording medium in one pass, then printing could be sped up by a factor equal to the number of passes.
Accordingly, there are xerographic systems, called "tandem" systems, that lay down all desired colors in a single pass over the surface of the photoreceptor. The architecture of a typical tandem system comprises several independent optical/xerographic subsystems running concurrently. Generally, there are as many subsystems as there are desired colors for printing. In a typical printing pass, each subsystem creates their latent image onto their dedicated photoreceptor simultaneously. The recording medium is circulated from subsystem to subsystem. As the recording medium passes each dedicated photoreceptor, the latent image is then transferred to the medium.
The main disadvantage in producing color images with a tandem system is cost. Each separate subsystem duplicates the number of optical elements. These additional optical elements add to the cost of the system. It would be more cost effective if a single photoreceptor could be addressed by multiple wavelengths using a common optical path.
Using a single optical path and a single photoreceptor, however, necessitates that the photoreceptor apparatus be sensitive to several different wavelengths simultaneously. One system provides a layered photoreceptor whereby each layer reacts to a specific wavelength and passes the remaining wavelengths to the successive layers below. Such a photoreceptor and system are described in U.S. patent applications Ser. Nos. 07/987,886 and 08/000,349 to Kovacs et al, filed on Dec. 9, 1992 and Jan. 4, 1993 respectively and assigned to Xerox Corporation and also in U.S. patent application Ser. No. 07/987,885 to Kovacs, filed on Dec. 9, 1992 and assigned to Xerox Corporation. These application are herein incorporated by reference.
A problem arises when using one optical path for several different -wavelengths of light. When all the various wavelengths pass through an aperture whose size is common to all wavelengths, .the spots of different wavelengths that form on the surface of a photoreceptor vary in size directly with wavelength. Thus, as the wavelength increases, so does the size of the imaging spot. This represents a problem in a laser printer which typically employs a single optical path consisting essentially of lenses and mirrors.
For most printing applications, the size of the imaging spot should be equal for all wavelengths. Otherwise, the final printout will be other than what the user intended. There is no logical reason why a red spot should always be larger than a blue one from a user's perspective. For other applications, it might be desirable to specify different spot sizes for different wavelengths. For example, a user may desire larger spots for the color blue than red. Additionally, the user may want to specify the exact size of the spot.
Thus, there has been a need to create an optical path that does not scale the number of optical elements according to the number of discrete wavelengths while, at the same time, produces spots of a specified size without regard to wavelength.
It is thus an object of the present invention to provide a single optical path for multiple wavelengths that produces specified spot sizes.